1. Field of the Invention
The present invention relates to an optical pickup apparatus that performs an operation of reading a signal recorded in an optical disc or an operation of recording a signal in the optical disc.
2. Description of the Related Art
There is known an optical disc device capable of a signal reading operation and signal recording operation by applying laser light emitted from an optical pickup apparatus to a signal recording layer of an optical disc.
The optical disc devices using optical discs called CDs or DVDs are known in general, however, optical discs whose recording densities are improved, that is, those using Blu-ray standard or HD DVD (High Density Digital Versatile Disc) standard optical discs have recently been developed.
Infrared light with a wavelength of 780 nm is used as the laser light performing the operation of reading a signal recorded in a CD standard optical disc, and red light with a wavelength of 650 nm is used as the laser light performing the operation of reading a signal recorded in a DVD standard optical disc.
A protective layer provided between an upper surface of the signal recording layer, that is, the signal recording layer, and a surface on which the laser light is incident, has a thickness of 1.2 mm in the CD standard optical disc; and the numerical aperture of an objective lens used for the operation of reading a signal from this signal recording layer is specified to 0.45. A protective layer provided on the upper surface of the signal recording layer has a thickness of 0.6 mm in the DVD standard optical disc, and the numerical aperture of the objective lens used for the operation of reading a signal from this signal recording layer is specified to 0.6.
In contrast to cases of such CD-standard and DVD-standard optical discs, laser light with a short wavelength, e.g., blue-violet light with a wavelength of 405 nm, is used as the laser light performing the operation of reading a signal recorded in the Blu-ray standard or HD DVD standard optical discs.
A protective layer provided on an upper surface of a signal recording layer has a thickness of 0.1 mm in the Blu-ray standard optical disc, and the numerical aperture of the objective lens used for the operation of reading a signal from this signal recording layer is specified to 0.85.
On the other hand, a protective layer provided on an upper surface of a signal recording layer has a thickness of 0.6 mm in the HD DVD standard optical disc, and the numerical aperture of the objective lens used for the operation of reading a signal from the signal recording layer is specified to 0.65.
As mentioned above, since the blue-violet light with a wavelength of 405 nm can be used as the laser light for performing the operation of reading a signal recorded in the Blu-ray standard or HD DVD standard optical disc, an optical pickup apparatus capable of performing the operations of reading signals from the optical discs of both standards can be made by sharing the same laser diode therebetween.
However, in order to read signals from both optical discs, the locations of their signal recording layers are greatly different from each other and the required numerical apertures of the objective lenses are also greatly different, and thus, it is necessary to switch the numerical aperture according to the optical disc, and an optical pickup apparatus capable of such an operation has been developed (See Japanese Patent Laid-Open Publication No. 2006-172605).
Such optical disc devices that can use not only the above-mentioned CD standard and DVD standard optical discs but also Blu-ray standard and HD DVD standard optical discs have recently been commercialized. Optical pickup apparatuses used in such optical disc devices are naturally configured to be able to perform an operation of reading a signal from a signal recording layer provided on an optical disc of a usable standard or an operation of recording a signal in the signal recording layer.
In such an optical pickup apparatus, since it is difficult to apply laser light with the above-mentioned wavelengths to the signal recording layers of the optical discs with a single objective lens, two objective lenses are used which are an objective lens for applying the laser light to the CD standard and DVD standard optical discs, for example, and an objective lens for applying the laser light to the Blu-ray standard optical disc, for example, (See Japanese Patent Laid-Open Publication No. Heill-23960).
As mentioned above, since the blue-violet light with a wavelength of 405 nm can be used as the laser light for performing the operation of reading a signal recorded in the Blu-ray standard or HD DVD standard optical disc, the optical pickup apparatus that can perform the operations of reading signals from the optical discs of both standards can be made by sharing the same laser diode therebetween.
In such optical discs of the above standards, not only that the thicknesses of the protective layers are different therebetween, but also the numerical apertures of the objective lenses are different, and thus, the optical pickup apparatus using two objective lenses respectively corresponding to the optical discs in the above standards has been developed.
In the optical pickup apparatus including the two objective lenses, a first objective lens and a second objective lens are fixed onto a lens holder supported so as to be capable of moving in a signal-surface direction of an optical disc and a radial direction of the optical disc by means of a supporting wire, and are configured so that a focus control operation for focusing the laser light as a spot onto the signal recording layer included in the optical disc and a tracking control operation for making the spot follow a signal track provided in the signal recording layer are performed by a movement operation of the lens holder.
A configuration is made such that the focus control operation and the tracking control operation in the optical pickup apparatus are performed by applying to a photodetector laser light called return light reflected from the signal recording layer included in the optical disc and by using a focus error signal or a tracking error signal obtained from the photodetector.
The optical pickup apparatus including the two objective lenses is configured such that the focus error signal and the tracking error signal are generated for each of the optical discs through applying to the photodetector the return light obtained from each of the objective lenses, and in the optical pickup apparatus configured such that the photodetector is shared, there is a problem that if a mounting position of the photodetector is determined so that that the position is suitable for the return light obtained from one of the objective lenses, it is not an optimal position for the return light obtained from the other one of the objective lenses.
Moreover, an optical element called polarization beam splitter is used for dividing the laser light emitted from a single laser diode to be separately guided to the first objective lens and the second objective lens, and a configuration is made such that the above dividing operation by the polarization beam splitter is performed through polarizing the laser light.
That is, a configuration is made such that in the case of an operation of reproducing a signal recorded in a first optical disc with the laser light focused by the first objective lens, the laser light of P-polarized light, which is linearly polarized light, is made to enter the polarization beam splitter, for example, so as to be passed through or reflected by a control film included in the polarization beam splitter in a direction of the first objective lens, while in the case of an operation of reproducing a signal recorded in a second optical disc with the laser light focused by the second objective lens, S-polarized light, which is linearly polarized light whose polarization direction is different by 90 degrees, is made to enter the polarization beam splitter so as to be reflected by or passed through the control film included in the polarization beam splitter in a direction of the second objective lens.
The optical pickup apparatus with the above configuration is configured so as to perform conversion into the P-polarized light or S-polarized light with a half-wave plate provided between the laser diode and the polarization beam splitter. A control element for performing such a polarization operation is expensive and increase in cost of the optical pickup apparatus is incurred, which is a problem.